1. Field of the Invention
The present invention relates to the application of coatings by electrodeposition and more particularly to the treatment of the electrodeposition bath to maintain initial bath properties.
2. Brief Description of the Prior Art
Electrodeposition has become a widely commercially accepted industrial coating technique. The coatings achieved have excellent properties for many applications and electrodeposition results in a coating which does not run or wash off during baking. Virtually any conductive substrate may be coated by electrodeposition, the most commonly employed substrates being metals.
In the electrodeposition process, the articles to be electrocoated are immersed in an aqueous dispersion of solubilized, ionized, film-forming materials such as synthetic organic vehicle resins. An electric current is passed between the article to be coated, serving as an electrode, and a counter electrode to cause deposition of a coating of the vehicle resin on the article. The article is then withdrawn from the bath, usually rinsed and then the coating either air-dried or baked in the manner of a conventional finish.
A major problem in the continuous electrodeposition process has been the control of the electrodeposition bath to maintain initial bath properties. One problem is that the bath often tends to become contaminated with iron and other metals. In the case of iron, the source of this contamination can be ferrous metal electrodes used in the electrodeposition process or parts of the articles being coated which may remain in the bath.
In the case of contamination with iron and other metals such as zinc, cadmium, copper, magnesium and calcium, it is believed that such contamination may result in a tendency of the resulting coating to be degraded by ultraviolet light. It is, therefore, the object of the method of the present invention to provide a means to treat electrodeposition baths to reduce or eliminate the tendency of the finished coating to degrade under such conditions.